Downhole cutting tool

ABSTRACT

A rotary tool for milling tubing in a borehole comprises at least one cutter with a cutter body and a cutting surface on the body. Each cutter is shaped and positioned on the tool so as to reduce tensile stress in the cutter, thereby reducing risk of the cutter becoming chipped or broken in use and produces swarf of reduced rigidity, less likely to form a blockage in the borehole.

CROSS-REFERENCE TO RELATED APPLICATION

The present document is based on and claims priority to GBNon-Provisional Application Serial No.: 1513927.2, filed Aug. 6, 2015,which is incorporated herein by reference in its entirety.

BACKGROUND

Oil and gas wells are usually lined with steel tubing which is cementedin place and forms a casing. Other steel tubing may be located insidethe casing. Some operations carried out within a well require theremoval of a length of steel tubing which has been secured within theborehole. This is customarily carried out using a tool referred to as amill which is used to mill out a length of tubing at a subterraneanposition which may be some distance from the surface. The mill cuts intothe tubing and comminutes it to swarf.

Various types of mill are used in boreholes. A mill for removing alength of tubing is commonly referred to as a section mill. It has thecharacteristic that it cuts away tubing as it moves along the tubing.Parts of the tool, or parts of the tool string which incorporates thetool, may extend into the tubing which has not yet been removed andthereby guide the tool to progress axially along the tubing as it isadvanced. This functionality contrasts with a window mill whose functionis to cut outwardly through tubing and start a new borehole branchingfrom an existing borehole.

A section mill which is able to mill out a length of tubing may have arotatable body with one or more projecting or expandable parts which maybe referred to by various names including blades and knives. Theseprojecting or expandable parts carry cutters of hard material, oftentungsten carbide, which cut into the tubing. The cut swarf iscustomarily entrained in the circulating flow of drilling fluid whichcarries it to the surface. However, pieces of swarf can becomingentangled within the borehole and form a blockage, sometimes referred toas a “bird's nest”, which can necessitate time-consuming interruption ofthe milling operation and removal of tools from the borehole in order toclear the blockage.

Another problem which can arise is damage to the cutters fitted to thetool. Wear of cutters during use of the milling tool is normal but it ispossible for cutters to become chipped or broken which reduces theefficiency and working life of the tool.

When a hard cutting tool acts on a workplace, the cutting surface may beheld perpendicular to the direction of traverse of the tool relative tothe workpiece or at an angle to the perpendicular. This angle to theperpendicular is referred to as a rake angle. A rake angle may bereferred to as forward or back, positive or negative, and the literatureis not consistent in use of this terminology. In the present disclosure,when the edge of the cutting surface which is in contact with theworkpiece is trailing behind the remainder of the cutting surface, thecutter is said to have a back rake also sometimes referred to as apositive rake.

SUMMARY

This summary is provided to introduce a selection of concepts that arefurther described below in the detailed description. This summary is notintended to be used as an aid in limiting the scope of the claimedsubject matter.

In a first aspect of the present disclosure, a rotary tool for millingtubing in a borehole comprises at least one cutter with a cutter bodyand a cutting surface on the body, where the cutter is shaped andpositioned on the tool such that:

at least part of the cutting surface is back raked, that is to say it isinclined relative to the direction of rotation with an edge where thecutting surface cuts furthest into the tubing being a trailing edge ofthe cutting surface relative to the direction of rotation,

at least part of the back raked cutting surface extends from the saidedge with a back rake angle which is from 15 to 60 degrees and at thesaid edge has an angle greater than 90 degrees included between thecutting surface and the surface of the cutter body following the cuttingsurface.

Because the rake angle between the cutting surface or part of thecutting surface and a perpendicular to the direction of traverserelative to the workpiece (i.e. direction of rotation relative to thetubing) lies in a range from 15 to 60 degrees, the angle between thecutting surface or part thereof and the direction of rotation lies in arange from 30 to 75 degrees.

We have found that a cutting surface with a significant back rake angleleads to the formation of swarf with less rigidity. It may be in theform of short pieces weakly connected together, or sometimes notconnected at all. Changing the nature of the swarf reduces the risk ofentangled swarf forming a “birds nest” blockage in the borehole. Asignificant back rake may require the cutter to be pressed against thetubing with more force than would be required with less back rake ornone. In a machine-shop context, a requirement for increased forcebetween a cutting tool and workpiece would be a disadvantage, but wehave recognized that when operating a cutting tool in a wellbore, arequirement for greater force is beneficial. More force can be providedby increasing the weight on the tool and control of the cutting speed byvarying the weight on the tool becomes easier. Increasing the includedangle between the cutting surface and a surface of the body behind thecutter surface makes the cutter more robust and reduces the risk of thecutter being chipped or broken.

The angle of the back rake may be such that the rake angle between thecutting surface or part thereof and a perpendicular to the direction ofmotion relative to the tubing is in a range from 20 or 25 degrees to 45or 50 degrees. In this case the angle between the cutting surface andthe direction of rotation will lie in a range from 40 or 45 degrees to65 or 70 degrees. The surface of the cutter body trailing back from theedge of the cutting surface may be aligned with the direction ofrotation or may be at a small angle to the direction of rotation. Thusit may be at an angle of 0 to 10 or 15 degrees to the direction ofrotation. In this case, the angle included between this surface of thecutter body and the cutting surface will be at least 95 degrees.

The cutter body may be dimensioned such that the at least part of theback raked cutting surface extends at least 1 mm from the said edgewhere the cutting surface cuts furthest into the tubing and the cutterbody's surface trailing back from the said edge extends at least 2 mmpossibly at least 3 mm or at least 5 mm back from the said edge.

An individual cutter body may be formed from a hard material other thandiamond. The hardness may be defined as a hardness of 1800 or more onthe Knoop scale or a hardness of 9 or more on the original Mohs scale(where diamond has a Mohs hardness of 10).

The rotary tool may comprise one or more supporting structures, eachhaving a plurality of cutters partially embedded in one of thesupporting structures. A cutter body may have a front which is exposedand a thickness extending into the support behind the exposed front. Thethickness dimension may be at least half, or at least three-quarters ofany distance across the cutter body transverse to the thickness. Theportion of the body which is embedded in the support structure may begreater than the volume of any portion projecting forwardly from thesupport structure. The cutting surface may extend as a bevel betweenside and front faces, at an angle (other than a right angle) to both andof course such that the rake angle between the cutting surface or partthereof and a perpendicular to the direction of motion relative to thetubing is in the range from 15 to 60 degrees.

Such a supporting structure may be an element which projects or isextensible outwardly from a central structure of the tool. The rotarytool may be constructed with a central structure for insertion axiallyinto the tubing, and at least one element which carries at least onesaid cutter and which projects outwardly from the central structure tobring the at least one cutter into contact with the tubing. There may bea plurality of such elements which are distributed azimuthally around alongitudinal axis of the tool.

In some embodiments the at least one element is expandable outwardly, byoperation of a mechanism within the tool structure. This can allow thetool to be inserted to a desired depth into a borehole and then expandedto begin cutting into tubing.

In another aspect of the present disclosure, a method of removing alength of tubing in a borehole comprises inserting into the tubing arotary milling tool which comprises a structure extending axially and atleast one element which projects or is extensible from the toolstructure and carries at least one cutter as set out above, and thenadvancing the tool axially while rotating the tool with the at least onecutter cutting into the tubing completely around the tubing. The cuttermay cut into a sidewall of the tubing or into an end face created by aninitial cut through the tubing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 diagrammatically illustrates a hard cutter carried on a rotarytool and in contact with tubing;

FIG. 2 shows equivalent stress distributed within a workpiece, in thedirection of traverse of a cutter, as predicted by finite elementanalysis, when the cutter has zero rake angle;

FIG. 3 shows equivalent stress distributed within a workpiece when thecutter has a rake angle of 30 degrees and when overall force betweenworkpiece and cutter is maximum;

FIG. 4 is similar to FIG. 3 but shows the distribution of equivalentstress when overall force between workpiece and cutter is minimum;

FIGS. 5 and 6 are similar to FIGS. 3 and 4, with a cutter rake angle of45 degrees;

FIG. 7 is similar to FIG. 3 with a cutter rake angle of 15 degrees;

FIG. 8 is similar to FIG. 4 with a cutter rake angle of 15 degrees, butshown at a larger scale;

FIG. 9 diagrammatically shows a piece of swarf produced by a cutter withrake angle of 45 degrees;

FIG. 10 is an enlarged cross section through a piece of swarf producedby a cutter with rake angle of 45 degrees;

FIG. 11 is an enlarged cross section through a piece of swarf producedby a cutter with zero rake angle;

FIGS. 12 to 15 show distribution of principal tensile stress withincutters with rake angles of zero, 15, 30 and 45 degrees respectively, aspredicted by finite stress analysis;

FIGS. 16, 17 and 18 are front and side views of cutters;

FIG. 19 shows a section mill with a cutter assembly retracted;

FIG. 20 is a cross section on line A-A of FIG. 19;

FIG. 21 shows the section mill of FIG. 19 with the cutter assemblyextended;

FIG. 22 is a cross-sectional elevation view of another embodiment ofexpandable tool, showing its expandable cutter blocks in retractedposition;

FIG. 23 is a cross-sectional elevation view of the expandable tool ofFIG. 19, showing the cutter blocks in expanded position;

FIG. 24 is a perspective view of a cutter block for the expandable toolof FIGS. 22 and 23;

FIG. 25 shows the cutter block of FIG. 24 in use;

FIG. 26 is a cross section on line B-B of FIG. 25; and

FIG. 27 is an enlarged detail from FIG. 25.

DETAILED DESCRIPTION

FIG. 1 shows part of the body of a cutter 10 which is carried onsupporting structure (not seen in this enlarged diagram) which is partof a rotary tool and brings the cutter body 10 into contact with theinside wall of tubing 12 so that part of the tubing is cut away as thecutter is driven in the direction of rotation of the tool around itsaxis, indicated by the chain dotted arrow 14. The direction of rotationis of course perpendicular to a radius 16 extending from the tool axis.

The cutter body has a flat front face 20 and a side surface 22 whichextends backwards relative to the direction of rotation, connected bybevelled face 24 which constitutes the cutting surface. The edge 26 ofthe cutting surface, cutting most deeply into the tubing 12, is trailingrelative to the parts of the cutting surface 24 which are not in contactwith tubing 12. The cutting surface 24 is thus positioned with a backrake angle. The rake angle between the cutting surface 24 and the radius16 is indicated 34. The angle between the cutting surface 24 and thedirection of rotation 14 is indicated 32.

The side surface 22 of the cutter body 10, extending back from the edge26 of the cutting surface 24, is inclined so as to diverge from thenewly-cut surface at a small angle 36 to the direction of rotation 16,so that the parts of the cutter body 10 behind the cutting surface 24 donot contact the freshly cut surface 28 on the tubing 12. The overallincluded angle between the cutting surface 24 and the side surface 22extending back from the edge 26 is the sum of the angles 34 and 38.

In accordance with the concepts set out above, the angle between thecutting surface 24 and the radius 16 lies in a range from 15 to 60degrees. The sum of the angles 32 and 34 is a right angle, because thedirection of rotation 14 and the radius 16 are perpendicular. Thus theangle 32 lies in a range from 30 to 75 degrees. In the embodimentillustrated the rake angle 34 lies in a narrower range from 20 to 50degrees and may be approximately 30 degrees.

The behaviour of a cutter made of hard material cutting into steel, withvariations in rake angle, was investigated by finite element analysis,which is a computational modelling procedure, assuming a constant depthof cut of 0.25 mm and a constant speed of traverse of the tool relativeto the steel workpiece of 1 metre/sec. The analysis ran for a period of1 millisecond corresponding to a distance traversed of 1 mm which isfour times the depth of cut. This analysis predicted that as the rakeangle of the tool is increased (and consequently the angle between thecutting surface and the direction of traverse decreases) the forcerequired to drive the tool increases and moreover with angles of 15degrees or more the force oscillates between upper and lower values. Thevalues predicted by this procedure were:

In direction of Perpendicular to traverse traverse Mean Amplitude ofMean Amplitude of Mean Rake force oscillation force oscillation periodof angle (kN) (kN) (kN) (kN) oscillation zero 1.9 0.1 0.2 0.1 <0.1 152.3 0.2 0.8 0.2 0.1 30 2.6 0.5 1.75 0.25 0.2 45 2.75 0.75 2.8 0.8 0.6

This analysis also provided visual maps of the workpiece showinginternal distribution of the equivalent stress in the direction oftraverse. Such maps are shown by FIGS. 2 to 8. In FIGS. 2 to 7 areas 40with lighter shading have equivalent stress between 730 MPa and 1150MPa, areas with heavier shading have equivalent stress in a range from1150 MPa to 1250 MPa and areas of the workpiece shown without shadingare subjected to equivalent stress of less than 730 MPa. (FIG. 8 haslarger scale and slightly different presentation as will be mentionedbelow). The stress results in strain and these maps of the workpieceshow deformation of material by the cutter.

FIG. 2 exemplifies the distribution of stress when the rake angle of thecutter 10 is zero. The distribution of stress varied during the lengthof time covered by the analysis, but a region 42 of highest stressbetween the arrows 44 was present throughout the duration of theanalysis.

FIGS. 3 and 4 show the predicted distribution of stress when the rakeangle was 30 degrees. FIG. 3 shows the predicted distribution whenapplied force in the direction of traverse was at the maximum(2.6+0.5=3.1 kN) and FIG. 4 shows the predicted distribution whenapplied force was at the minimum (2.6-0.5=2.1 kN). As shown in FIG. 3,when the force in the direction of traverse is at a maximum, there is aregion 42 of highest stress between the areas 44. By contrast, when theforce in the direction of traverse is at a minimum, not only is thestress within the workpiece 12 generally lower, there is also alocalised region 46 of stress below 730 MPa along a line between arrows44.

The interpretation of the oscillation in applied force, tabulated above,and the finite element analysis shown by FIGS. 2-4 is that with zerocutter rake angle, equivalent stress in the workpiece 12 remains fairlyconstant and the material which is cut from the workpiece (the so-calledchip) remains as a continuous strand even though it undergoes plasticdeformation as it is cut. By contrast, with the 30 degree rake angle,stress and strain in the region of the workpiece 12 between arrows 44increase until thermal softening of the material (resulting from plasticdeformation causing a localised increase in temperature within theworkpiece) enables a region of reduced stress between arrows 44 topropagate from the edge of the cutter to the free surface of theworkpiece. When this thermally softened region has fully propagated, thechip being cut is displaced. This gives a temporary reduction of stressin the workpiece 12 and the region 46 of low stress between the arrows44. Thus the chip becomes separated from the workpiece along the linebetween arrows 44.

FIGS. 5 and 6 show the predicted distribution of stress when the cutterrake angle was 45 degrees and applied force in the direction of traversewas at the maximum (3.5 kN) and minimum (2.0 kN) respectively. FIG. 5shows a region 42 with the high level of stress located between thearrows 44 when the applied force in the direction of traverse is at themaximum, and (similarly to FIG. 4) a region 47 with low level of stressextending along the line between arrows 44, which is where the chipseparates from the workpiece 12.

FIGS. 7 and 8 show the distribution of stress predicted by finiteelement analysis when the cutter rake angle was 15 degrees and appliedtransverse force between cutter and workpiece was maximum and minimumrespectively. There was qualitative similarity to the maps at cutterrake angles of 30 and 45 degrees. As shown by FIG. 7, when applied forcein the direction of traverse was at the maximum (2.3+0.2=2.5 kN) therewas a region of high equivalent stress 42 between the arrows 44. FIG. 8shows the distribution of stress when the applied force in the directionof traverse was at the minimum (2.3−0.2=2.1 kN). This drawing is at alarger scale than FIGS. 2 to 7. Areas of light shading 41 have stressbetween 730 MPa and 1000 MPa. Areas 43 have stress between 1000 MPa and1150 MPa. Areas 42 have stress between 1150 MPa and 1250 MPa. FIG. 8shows that when applied force in the direction of traverse was at theminimum, a region 48 of stress not exceeding 1000 MPa extends along theline between the arrows 45, in between areas 43 of stress exceeding 1000MPa within which there were regions 42 of higher stress. Levels ofstress within the region 48 were almost all below 900 MPa.

The predictions of finite element analysis shown by FIGS. 5 and 6 wereconfirmed by experiment. A cutter as illustrated by FIG. 1, with a rakeangle of 45 degrees was used to cut material from a steel workpiece. Thechip took the form shown diagramatically by FIG. 9. It consisted of aribbon of short sections 50 weakly joined one to the next along lines52. Each of these lines 52 correspond to the instant when a section 50detached from the workpiece 12 and the localised heating of thedisplaced material allowed a weak weld to form between adjacent sections50. A piece of the swarf was embedded in rigid resin, then cut through,polished and photographed under high magnification. This photographshowed a cross section of the swarf along a line such as line X-X inFIG. 9. The photograph was digitally manipulated to show the edge of theswarf: the result is shown as FIG. 10. A double headed arrow shows adistance of 0.1 mm. As a comparison, a piece of swarf obtained with acutter of zero rake angle was likewise embedded in rigid resin, cutthrough, polished and photographed under high magnification. Thisphotograph was likewise digitally manipulated to show the edge of theswarf: the result is shown as FIG. 11. It can be seen that the swarf inFIG. 11 was a continuous strip whereas the swarf in FIG. 10 consisted ofa number of sections 50, partially separated by gaps 54 so that thejoins between the sections 50 do not extend across the full thickness ofthe piece of swarf.

Finite element analysis was also applied to stress within the cutters,as shown by FIGS. 12 to 14. Each of these illustrates the distributionof maximum principal tensile stress (the scalar value of maximum tensilestress regardless of direction). Regions 56 where the tensile stressexceeds 100 MPa are shown with shading. In the case of the cutter withzero rake angle shown by FIG. 12 the region 56 included a small region58 adjoining the cutting surface in which the principal stress was from300 MPa up to 370 MPa. For the cutter with rake angle of 15 degrees sothat the included angle at edge 26 is 100 degrees, the area 56 as shownby FIG. 13 had maximum stress of 230 MPa. In the case of the cutterswith 30 and 45 degree cutter rake angles, such that the included anglesat the edge 26 were 115 and 130 degrees the principal stress in theareas 56 did not exceed 170 MPa and 130 MPa respectively. Cutterbreakage is likely to start where material is under tension and so thelowering of tensile stress with increasing rake angle is valuablebecause it reduces the risk of cutter breakage during use. In addition,a larger included angle results in a stronger edge and longer tool life.

FIGS. 16, 17 and 18 show three embodiments of cutters. FIGS. 16a, 17aand 18a are front views, FIGS. 16b, 17b and 18b are side views. Thecutter of FIG. 16 is rectangular. A cutting surface 60 at an angle of 30degrees to the front face 61 extends to the edge 62 which, when thecutter is mounted on a tool, is the edge where the cutter is furthestinto the material being cut. The angle included at edge 62 is 120degrees. The cutter of FIG. 17 has a cylindrical body 63 and a frontface 64 with smaller diameter surrounded by an annular surface 65 at anangle of 45 degrees to the front face 64. When mounted on a tool, partof this annular surface 65 is the cutting surface. The angle includedbetween the side wall of the cutter body 63 and the surface 65 is 135degrees, as shown. The thickness of the cutter behind the front face isindicated at 66 and the cross section transverse to the thickness, whichin this case is the diameter of the cylindrical cutter body 63, isindicated 67. In this embodiment shown here, the thickness dimension 66is about 0.8 of the cross section 67.

FIG. 18 shows a cutter which also has a cylindrical body, but has afront face 64 which is eccentric relative to the main body. The widestpart 68 of the annular surface around the front face 64 is at an angleof 45 degrees to the front face 64 but the narrowest part 69 is at adifferent angle, as can be seen in the drawing. This cutter is mountedin a tool so that wider parts of the annular surface provide the cuttingsurface.

These cutters are made of a hard material which may be tungsten carbide.This hard material may be provided as a powder which is compacted intothe shape of the cutter and then sintered. Manufacturers of sinteredtungsten carbide cutters include Cutting and Wear Resistant DevelopmentsLtd, Sheffield, England and Hallamshire Hard Metal Products Ltd,Rotherham, England.

Tungsten carbide is commonly used for cutters because it is very hardand also has good thermal stability. Other hard materials which may beused are carbides of other transition metals, such as vanadium,chromium, titanium, tantalum and niobium. Silicon, boron and aluminiumcarbides are also hard carbides. Some other hard materials are boronnitride and aluminium boride. A hard material (which is other thandiamond) may have a hardness of 1800 or more on the Knoop scale or ahardness of 9 or more on the original Mohs scale (where diamond has aMohs hardness of 10).

FIGS. 19 to 21 show a section mill used to remove a length of tubing,starting at a subterranean location which is some way down a borehole.An existing borehole is lined with tubing 72 (the wellbore casing) andcement 74 has been placed between the casing and the surrounding rockformation. The tubing and cement may have been in place for some years.It is now required to remove a length of tubing, starting at a pointbelow ground. One possible circumstance in which this may be required iswhen a borehole is to be abandoned, and regulatory requirementsnecessitate removal of a length of tubing and surrounding cement inorder to put a sealing plug in place.

FIGS. 19 and 21 are cross-sectional elevations showing part of the toolto the right of chain dotted centre line CL-CL. FIG. 20 is a schematiccross section looking along the tool axis at the level of the arrows A-Ain FIG. 19, with plunger head 91 omitted. As shown by FIG. 19, the toolhas a cylindrical body with an outer wall 80. Three slots are formed inthis body at positions which coincide axially and distributedazimuthally around the tool axis. At either side of each slot there is aplate 81 extending inwardly from the wall 80. A cutter assembly, whichcomprises cutters attached to an arm 82 made of steel plate, isaccommodated within each slot. As can be seen from FIG. 19, each arm 82is pivoted to swing around a pin 83 supported by the plates 81. Each arm82 can swing from a retracted position shown in FIG. 19 to an expandedposition shown in FIG. 21. Expansion is brought about by a hydrauliccylinder and piston, not shown, operated by pressure of drilling fluidand connected to drive plunger shaft 89. Pressure of drilling fluidcauses the plunger shaft 89 to move downwardly. A domed plunger head 91on the end of shaft 89 acts on the inside edges of arms 82, forcing eacharm to pivot outwardly towards the position shown in FIG. 21. Outwardexpansion is limited by prolongations 92 of the arms 82 when theseprolongations abut the inside face of the tool wall 80 as indicated at93 on FIG. 21.

Each arm 82 has cutters 86, 87 of the type shown by FIG. 16 attached toits front face as seen in FIGS. 19 and 21 with the edges 62 of thesecutters aligned with the edges 84, 85 of the arm 82. The cutters 86, 87may be attached to the arm 82 by brazing and when attached to the arm 82the cutting surfaces 60 of the cutters have a back rake of 30 degrees.At the corner of the arm 82 there is a cutter 88 which extends aroundthe curve between the edges 84, 85 of the arm 82 and has a cuttingsurface with the same back rake of 30 degrees which also follows aroundthe curve.

For use the section mill is included in a drill string and lowered tothe point within the borehole tubing 72 where milling is to begin. Thedrill string is then rotated and the plunger head 91 is driven downwardsforcing the arms 82 outwards towards the position shown by FIG. 21. Thecutters 87 on the outer edges 85 of the arms 82 cut radially outwardsinto and through the tubing 72 until the arms are fully extended asshown in FIG. 21. The tool is then advanced axially downwards and thecutters 86 on the edge 84 progressively cut downwards into an end faceon the tubing 72, destroying a length of the tubing by milling it toswarf.

As the cutters 86 on an arm 82 cut into the tubing 72, their cuttingsurfaces are at an angle of 30 degrees to the plane of the arm 82. Thisarm extends axially and the axial direction is perpendicular to therotational direction of the tool and to the end face of the tubing 72which is being cut. The cutters therefore are at a back rake of 30degrees as they cut into the tubing 72. Previously, as the cutters 87were cutting into the inside face of the tubing 72, they also were at aback rake relative to the inside surface of the tubing, although theback rake angle relative to this surface will vary as the arms swingaround their pivots 83.

FIGS. 22 to 26 show a rotary tool which is an expandable milling tool,utilising an expansion mechanism which is already used in reamers. FIG.22 shows the tool with its expandable cutter blocks with the blocks inretracted position. FIG. 23 is a corresponding view with the blocks inexpanded position.

This expandable tool comprises a generally cylindrical tool body 106with a central flowbore 108 for drilling fluid. The tool body 106includes upper 110 and lower 112 connection portions for connecting thetool into a drilling assembly. Intermediately between these connectionportions 110, 112 there are three recesses 116 formed in the body 106and spaced apart at 120 degrees intervals azimuthally around the axis ofthe tool.

Each recess 116 accommodates a cutter block 122 in its retractedposition. The three cutter blocks may be identical in construction anddimensions. One such cutter block 122 is shown in perspective in FIG.24. Each block 122 is formed of a steel inner block part 124 with aprojecting lug 125 along its outer surface and an outer block part 126astride the lug 125 and bolted to the inner part 124 by bolts (notshown) inserted through the apertures 128 into threaded holes in theinner part 124. Details of the outer part 126 are not shown in FIGS. 22and 23 and will be described in more detail below. The radially outerface 129 of the outer block part 126 is indicated without detail inFIGS. 22 and 23.

The inner block part 124 has side faces with protruding ribs 117 whichextend at an angle to the tool axis. These ribs 117 engage in channels118 at the sides of a recess 116 and this arrangement constrains motionof each cutter block such that when the block 122 is pushed upwardlyrelative to the tool body 106, it also moves radially outwardly towardsthe position shown in FIG. 23 in which the blocks 122 project outwardlyfrom the tool body 106. The ribs 117 in channels 118 allow each cutterblock to move bodily upwardly and outwardly in this way without changingits orientation (i.e. without changing its angular position) relative tothe tool axis.

A spring 136 biases the block 122 downwards to the retracted positionseen in FIG. 22. The biasing spring 136 is disposed within a springcavity 138 and covered by a spring retainer 140 which is locked inposition by an upper cap 142. A stop ring 144 is provided at the lowerend of spring 136 to keep the spring in position.

Below the moveable blocks 122, a drive ring 146 is provided thatincludes one or more nozzles 148. An actuating piston 130 that forms apiston cavity 132 is attached to the drive ring 146. The piston 130 isable to move axially within the tool. An inner mandrel 150 is theinnermost component within the tool, and it slidingly engages a lowerretainer 170 at 172. The lower retainer 170 includes ports 174 thatallow drilling fluid to flow from the flowbore 108 into the pistonchamber 132 to actuate the piston 130.

The piston 130 sealingly engages the inner mandrel 150 at 152, andsealingly engages the body 106 at 134. A lower cap 180 provides a stopfor the downward axial movement of piston 130. This cap 180 isthreadedly connected to the body 106 and to the lower retainer 170 at182, 184, respectively. Sealing engagement is provided at 186 betweenthe lower cap 180 and the body 106.

A threaded connection is provided at 156 between the upper cap 142 andthe inner mandrel 150 and at 158 between the upper cap 142 and body 106.The upper cap 142 sealingly engages the body 106 at 160, and sealinglyengages the inner mandrel 150 at 162 and 164.

In order to expand the blocks 122, drilling fluid is directed to flowdownwards in flowbore 108. It flows along path 190, through ports 174 inthe lower retainer 170 and along path 192 into the piston chamber 132.The differential pressure between the fluid in the flowbore 108 and thefluid in the borehole annulus surrounding tool causes the piston 130 tomove axially upwardly from the position shown in FIG. 22 to the positionshown in FIG. 23. A portion of the flow can pass through the pistonchamber 132 and through nozzles 148 to the annulus as the cutter blocksstart to expand. As the piston 130 moves axially upwardly, it urges thedrive ring 146 axially upwardly against the blocks 122. The drive ringpushes on all the blocks 122 simultaneously and moves them all axiallyupwardly in recesses 116 and also radially outwardly as the ribs 150slide in the channels 118. The blocks 122 are thus driven upwardly andoutwardly in unison towards the expanded position shown in FIG. 23.

The movement of the blocks 122 is eventually limited by contact with thespring retainer 140. When the spring 136 is fully compressed against theretainer 140, it acts as a stop and the blocks can travel no further.There is provision for adjustment of the maximum travel of the blocks122. This adjustment is carried out at the surface before the tool isput into the borehole. The spring retainer 140 connects to the body 106via a screwthread at 186. A wrench slot 188 is provided between theupper cap 142 and the spring retainer 140, which provides room for awrench to be inserted to adjust the position of the screwthreaded springretainer 140 in the body 106. This allows the maximum expanded diameterof the reamer to be set at the surface. The upper cap 142 is also ascrewthreaded component and it is used to lock the spring retainer 140once it has been positioned.

The outer part 126 of each cutter block is a steel structure with sideface 200 which is the leading which is the leading face in the directionof rotation. An area 204 of this face is slanted back. This steel outerpart 126 incorporates cylindrical pockets which receive the cylindricalbodies of cutters of the type shown in FIG. 17. The cutters are held inplace by brazing. The front faces 63 and the surrounding surfaces 66 areexposed within the area 204.

The outward facing surface of the outer block part 126 comprises apart-cylindrical outward facing surface 221 with a radius such that thesurface 221 is centred on the tool axis when the cutter blocks are fullyextended. The cutter 211 is positioned so that its radially outer edgeis at the same distance from the tool axis as the surface 221. There isalso a part-cylindrical outward facing surface 222 which is further outfrom the tool axis and again is centred on the tool axis when the cutterblocks are fully extended. The edge of cutter 212 is at the samedistance from the tool axis as the surface 222. This pattern of a cutterand a part-cylindrical outward facing surface where the surface and theradial edge of the cutter are both at the same distance from the toolaxis is repeated along the block by cutter 213 and surface 223, cutter214 and surface 224 and so on at progressively greater radial distancesfrom the tool axis. Transitional surfaces 227 connecting adjacentsurfaces 221 and 222, similarly 222 and 223 and so on, have the samecurvature as, and are aligned with, the curved edges of cutters 211-216.

For use as a section mill, the tool is attached to a drill string andlowered into the borehole tubing 68 to the required depth. The drillstring is then rotated and the tool is expanded by pumping fluid intoflowbore 108 as described above. The radially outer edge of cutter 216contacts the interior face of the tubing 68 and cuts into it. Thisallows expansion to continue and the cutters 215 to 211 contact theinside face of the tubing in sequence, cutting into and through thetubing until the fully expanded position of the blocks is reached. Thetool is then advanced axially. This is illustrated by FIG. 25 whichshows the outer part 126 of a cutter block in use to remove tubing 68within a borehole. Numeral 107 indicates an edge of the outer wall ofthe tool body 106, exposed at the side of a recess 116. The tool is nowadvancing axially in the downward direction shown by arrow D. Theleading cutters 211 on each cutter block are positioned to any corrosionor deposits 252 and also remove some material from the inside wall ofthe tubing 250, thus exposing a new inward facing surface 254.

The amount of expansion of the tool is arranged such that when thecutter blocks are fully expanded, the surfaces 221 and the outerextremities of the leading cutters 211 are at a radial distance from thetool axis which is slightly greater than the inner radius of the tubing250 but less than the outer radius of the tubing. If necessary, theamount of expansion is limited by adjusting the screwthreaded springretainer 140 in the body 106, using a wrench in the wrench slot 188while the tool as at the surface so that expansion goes no further thanrequired.

The new internal surface 254 is at a uniform radius which is the radialdistance from the tool axis to the extremities of the leading cutters211. Because the part-cylindrical outward facing surfaces 221 of thethree blocks have a curvature which is centred on the tool axis and atthe same radial distance from the tool axis as the extremities of theleading cutters 211, they are a close fit to this surface 254 created bythe cutters 211, as is shown in FIG. 25, and act as guide surfaces whichslide over this new internal surface 254 as the tool rotates. The toolaxis is thus positioned relative to the tubing 250.

As the tool advances axially, the cutters 212 which extend outwardlybeyond the surfaces 221 remove the remainder of the tubing indicated at256 outside the new surface 254 so that the full thickness of the tubing250 has been removed. The cutters 213 to 216 cut through any cement orother material which was around the outside of the tubing.

Because the part-cylindrical surface 221 is centred on the tool axiswhen the cutter blocks are fully expanded, the tool is configured forremoving tubing of a specific internal diameter. However, the tool canbe used to remove tubing within a range of internal diameters bypreparation at the surface, before it is put into a borehole. The toolis configured by fitting the cutter blocks with outer parts 124dimensioned so that the radius of curvature of the surface 221 is thesame as or slightly larger than the original (i.e. as manufactured)internal radius of the tubing to be removed. Also, at the surface,spring retainer 140 is adjusted, using a wrench in slot 188, so thatexpansion of the tool is limited to the extent required, at which thecutters 211 create the new internal surface on line 254 and the surfaces221 are a close fit against this surface.

FIG. 26 is a cross section showing the cutter 211. As can be seen thepart of the face 66 close to the inside wall of the tubing is thecutting surface and is at a back rake angle of about 50 degrees. FIG. 27is an enlarged view of the face of the cutter 211. The cutting surfaceis an arc of the face 66 of the cutter, approximately between the chaindotted lines.

It will be appreciated that the embodiments and examples described indetail above can be modified and varied within the scope of the conceptswhich they exemplify. Proportions may be varied and in particular backraked cutting surfaces may be larger or smaller than shown in thedrawings. Features referred to above or shown in individual embodimentsabove may be used together in any combination as well as those whichhave been shown and described specifically. More particularly, wherefeatures were mentioned above in combinations, details of a feature usedin one combination may be used in another combination where the samefeature is mentioned. Accordingly, all such modifications are intendedto be included within the scope of this disclosure as defined in thefollowing claims.

1. A rotary tool for milling tubing in a borehole, the rotary toolcomprising: at least one cutter with a cutter body and a cutting surfaceon the body, wherein the cutter is shaped and positioned on the rotarytool such that at least part of the cutting surface is back rakedrelative to a direction of rotation so that the cutting surface cutsfurthest into the tubing at an edge which is a trailing edge of thecutting surface relative to the direction of rotation, and wherein atleast part of the back raked cutting surface extends from the edge witha rake angle between the cutting surface and a perpendicular to thedirection of rotation which is in a range from 15 to 60 degrees and, atthe edge of the cutting surface, has an angle greater than 90 degreesincluded between the cutting surface and the surface of the cutter bodyfollowing the cutting surface.
 2. The rotary tool of claim 1 wherein theat least part of the back raked cutting surface extends from the edgewith a rake angle which is in a range from 20 to 50 degrees.
 3. Therotary tool of claim 1 wherein, at the edge, the surface of the cutterbody following the at least part of the back raked cutting surfacediverges from the cutting surface at an angle of between 0 and 15degrees to the direction of rotation.
 4. The rotary tool of claim 1wherein the at least part of the back raked cutting surface extends fromthe edge with a rake angle which is in a range from 20 to 60 degrees andhas an angle of at least 100 degrees included between the at least partof the back raked cutting surface and the surface of the cutter bodyfollowing the cutting surface.
 5. The rotary tool of claim 1 wherein theat least one cutter comprises a cutter body of a hard material.
 6. Therotary tool of claim 5 wherein the hard material has a Knoop hardness of1800 or more.
 7. The rotary tool of claim 1, further comprising: acentral structure for insertion axially into the tubing, and at leastone element which carries the at least one cutter and which projects oris extensible from the central structure to bring the at least onecutter into contact with the tubing.
 8. The rotary tool of claim 7wherein the at least one element is configured to bring the at least onecutter into contact with an internal surface of the tubing to cutradially outwardly into the tubing.
 9. The rotary tool of claim 7wherein the at least one element has the at least one cutter with thecutter body partially embedded therein and partially exposed, such thatthe embedded portion of the cutter body is of greater volume than theexposed portion.
 10. The rotary tool of claim 9 wherein the at least onecutter has an exposed front and a partially embedded thickness followingthe exposed front with an extent which is at least half the length ofany dimension across the cutter body, perpendicular to the partiallyembedded thickness.
 11. The rotary tool of claim 7 wherein the rotarytool has a plurality of elements which each carry at least one cutterformed of hard material, which project or are extensible from the toolbody and which are distributed azimuthally around a longitudinal axis ofthe rotary tool.
 12. A method of removing a length of tubing in aborehole, the method comprising: inserting into the tubing a rotary toolfor milling tubing in a borehole, the rotary tool comprising at leastone cutter as defined in claim 1, and advancing the rotary tool axiallywhile rotating the rotary tool with the at least one cutter cutting intothe tubing completely around the tubing.
 13. The method of claim 12wherein the at least one cutter is carried by at least one elementconfigured to bring the at least one cutter into contact with aninternal surface of the tubing, whereby the at least one cutter cutsradially outwardly into the internal surface completely around thetubing.